Methods for detecting the endpoint of a photoresist stripping process

ABSTRACT

Methods for detecting the endpoint of a photoresist stripping process provide O for reaction with the photoresist for a wafer to be stripped of photoresist. NO is also supplied for reaction with O not reacted with the photoresist. After substantially all the photoresist is stripped from the wafer, the rate of a reaction of O and NO to form NO 2  increases, which increases the intensity of emitted light. An operation of detecting this increase in light intensity signals the endpoint of the photoresist stripping process.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This Application is a divisional of prior application Ser. No.09/468,742, from which priority under 35 U.S.C. Section 120 is claimed.The entire disclosure of the prior application from which a copy of thedeclaration is herewith supplied is considered as being part of thedisclosure of this Application and is hereby incorporated by referencetherein.

BACKGROUND OF THE INVENTION

[0002] This disclosure relates generally to photoresist strippingprocesses, and more particularly to a method and apparatus for detectingthe endpoint of a photoresist stripping process.

[0003] Fabrication of integrated circuits generally starts with a thinslice of silicon called a wafer. On this wafer one can fabricate severalhundred individual chips. Each chip may contain 10 to 20,000 componentsfor a total of several million devices per wafer. The devices includeresistors, diodes, capacitors and transistors of various types. Thefabrication of the devices includes depositing desired materials (suchas silicon dioxide, aluminum, etc.) at certain locations.

[0004] A technique called photolithography is used to facilitate theintroduction of materials at desired locations on the wafer and theremoval of undesired material at other locations. As an example, a layerof aluminum is first deposited on the wafer. Next, the wafer is coatedwith a light sensitive polymer called photoresist. A mask is used toexpose selected areas of photoresist to UV light. The UV light inducespolymerization in the exposed photoresist. UV light causes the exposedphotoresist to cross link, rendering it insoluble in developingsolution. Such a photoresist is called a positive photoresist. Anegative photoresist shows an opposite behavior. That is, exposure to UVmakes the photoresist soluble in developing solution. After the exposureto light, the soluble portions of the photoresist are removed, leaving apattern of photoresist.

[0005] Immediately after photolithography, the wafer with patternedphotoresist is aluminum etched to remove the aluminum where there is nopattern. This has the effect of transferring the pattern to thealuminum, creating electrical connections among devices at differentlocations.

[0006] After the aluminum etch process is complete, the photoresist isremoved from the wafer in a process called photoresist stripping. Thestripping of photoresist from the wafer surface must be essentiallycomplete, since photoresist left on the wafer surface will cause defectsin the integrated circuit. An important consideration in the photoresiststripping process is determining a time, referred to as the endpoint, atwhich to end the process. This time must be chosen so that thephotoresist is entirely removed from the wafer. Preferably, the timewill not exceed the time when the photoresist has been entirely removed,since this decreases the efficiency of the fabrication of integratedcircuits.

[0007]FIG. 1 is a flow chart of a prior art method for strippingphotoresist 2. The prior art method begins in a step 4. The photoresiststripping process includes introducing a flow of O atoms into astripping chamber that holds the wafer. The O atoms react with thephotoresist, removing the photoresist from the wafer. The products ofthis reaction are removed from the chamber in the flow of gases leavingthe chamber. The prior art method 2 continues the photoresist strippingprocess for a predetermined time in a step 6, and ends the photoresiststripping process in a step 8 after the predetermined time elapses. Thepredetermined time is chosen to ensure that enough time passes to ensurethat essentially all the photoresist has been stripped from the wafer.

[0008] One problem with the prior art method shown in FIG. 1 is that itprovides no means of detecting if all of the photoresist has beenstripped from the wafer. Even if the photoresist has not been completelystripped from the wafer, the prior art method still stops the strippingprocess after the predetermined time has elapsed. In such a case, thepresence of photoresist on the wafer will not be discovered until later,and the wafer will either have to be put through the photoresiststripping process again or discarded. Either alternative adds expenseand time to the fabrication of the integrated circuit.

[0009] Another problem with the prior art method shown in FIG. 1 is thatthe time of the photoresist stripping process is predetermined. As such,the time may not be optimally efficient. The time may be too short, inwhich case some or all of the wafers fail to be completely stripped ofphotoresist, requiring further processing of the wafers. Alternatively,the time may be too long, in which case the process continues after allthe photoresist has been stripped from the wafer. This decreasesthroughput and fabrication efficiency.

[0010] It would therefore be desirable to provide a method and apparatusfor detecting when essentially all the photoresist has been strippedfrom a wafer during the photoresist stripping process. Such a method andapparatus would preferably increase wafer fabrication throughput andefficiency.

SUMMARY OF THE INVENTION

[0011] Disclosed embodiments provide a method and apparatus fordetecting the endpoint for a photoresist stripping process. Preferably,O and NO are introduced into the stripping chamber. When O and NO react,they produce NO₂ and emit light. However, while photoresist remains onthe wafer, the O that is introduced mostly reacts with the photoresistand only a small amount of light is emitted from the O+NO reaction.After essentially all the photoresist has been stripped from the wafer,much of the O that would have reacted with the photoresist reacts withthe NO instead. Thus, the rate of the O+NO reaction increases and theamount of light produced in the reaction increases. The method andapparatus detects the light emitted from the reaction of O and NO anduses the increase in the light emission levels to detect the endpoint ofthe photoresist stripping process.

[0012] According to a preferred embodiment, the apparatus includes astripping chamber, a wafer disposed within the stripping chamber, and alight detecting apparatus for monitoring the intensity of light emittedwithin the stripping chamber. The light detecting apparatus detects theintensity of light emitted from the reaction of O and NO to form NO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The disclosed embodiments will be better understood whenconsideration is given to the following detailed description inconjunction with the accompanying drawings, with like reference numeralsdesignating like elements.

[0014]FIG. 1 is a flow chart of a prior art method for strippingphotoresist.

[0015]FIG. 2 is a flow chart of a method for stripping photoresist.

[0016]FIG. 3 is a flow chart showing the process of photoresiststripping according to an embodiment of the present invention.

[0017]FIG. 4 is a schematic view of a first embodiment.

[0018]FIG. 5 is a schematic view of a second embodiment.

[0019]FIG. 6 is a schematic view of a third embodiment.

[0020]FIG. 7 is a graph showing the intensity of light detected from thereaction of O and NO.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS

[0021]FIG. 2 shows a flow chart depicting the major steps of a method200 for detecting the endpoint of a photoresist stripping process. In aninitial step 202, the photoresist stripping process begins. Thephotoresist stripping process is detailed further in FIG. 3 and itsaccompanying description.

[0022] In a step 204, an increase in emitted light is detected.Preferably, the light is emitted from the reaction of O and NO to formNO₂. Suitable choice of an apparatus to detect the increase in emittedlight, and suitable placement of the apparatus to detect the increase inemitted light are well within the capabilities of those skilled in theart of wafer processing. The increase in emitted light signals theendpoint of the photoresist stripping process.

[0023] The photoresist stripping process continues for a predeterminedtime in a step 206. In a preferred embodiment, the predetermined time istwenty seconds. While the increase in light emitted signals thatsubstantially all of the photoresist has been stripped from the wafer,the extra time for the photoresist stripping process ensures that thewafer has been sufficiently stripped of photoresist.

[0024] Finally, in a step 208, the photoresist stripping process isended. The photoresist has been stripped from the wafer and the wafer isready for further processing.

[0025]FIG. 3 shows a flow chart depicting the photoresist strippingprocess 300 of a preferred embodiment. The photoresist stripping process300 begins in a step 302. As the process 300 begins, O and NO areintroduced into the stripping chamber in a step 304. In otherembodiments, NO is not introduced into the chamber, but is introduceddownstream from the stripping chamber outlet, into a flow of gas comingfrom the stripping chamber. In a preferred embodiment, the O and NO areproduced by introducing O₂ and N₂ into a plasma chamber, dissociatingthe O₂ and N₂ within the plasma chamber so that a flow of O and NO formsand enters the stripping chamber, in such a manner that substantially noplasma enters the stripping chamber.

[0026] In a step 306, the photoresist is stripped from a wafer. Thisoccurs by a reaction of O with the photoresist, which removes thephotoresist from the wafer. While the photoresist stripping occurs, mostof the O introduced into the stripping chamber is consumed by thereaction with the photoresist. Only a small amount is available tocombine with NO to create NO₂ and emit light. Thus, only a small amountof light is emitted.

[0027] In a step 308, the photoresist stripping is completed. At thispoint substantially all of the photoresist has been removed from thewafer. Since the photoresist has been removed, the O no longer primarilyreacts with the photoresist. As seen in a step 310, the O is nowavailable to combine with NO to create NO₂ and emit light. The increasedavailability of O increases the reaction of O and NO to create NO₂ andlight. Thus, the intensity of emitted light increases. This increase inemitted light is a visible signal, as seen in a step 312. The signal isobserved and used to signal the endpoint of the photoresist strippingprocess in a step 314.

[0028]FIG. 4 is a schematic view of a first embodiment of an apparatus10. A plasma chamber 12 comprises an inlet 14 and an outlet 16. In apreferred embodiment, a flow of O₂ and N₂ enters the plasma chamber 12through the plasma chamber inlet 14. Within the plasma chamber 12, theO₂ and N₂ are dissociated so that NO and O are formed. The flow of NOand O exits the plasma chamber 12 through the plasma chamber outlet 16.A stripping chamber 18 comprises an inlet 20 and an outlet 22. Theplasma chamber outlet 16 is in fluid communication with the strippingchamber inlet 20. Thus, the flow of NO and O enters the strippingchamber 18 as it leaves the plasma chamber 12. Substantially no plasmaenters the stripping chamber 18 from the plasma chamber 12, onlyuncharged gas.

[0029] A wafer 24, at least partially coated with a layer of photoresist26, is disposed inside the stripping chamber 18. As the flow of O and NOpasses through the stripping chamber 18, the O reacts with the layer ofphotoresist 26 and removes the layer of photoresist 26 from the wafer24. Inside the stripping chamber 18, O reacts with NO to form NO₂ andemit light. However, while the layer of photoresist 26 remains on thewafer 24, much of the O is consumed by reacting with the layer ofphotoresist 26. Little O is left over to react with NO, so little lightis emitted. When the layer of photoresist 26 is substantially entirelyremoved from the wafer 24, the O is no longer consumed by a reactionwith the layer of photoresist 26. The O now reacts with NO. More Oreacts with NO after the layer of photoresist 26 is removed, so morelight is emitted from the reaction of O and NO to form NO₂. Therefore,the amount of emitted light increases after the layer of photoresist 26has been essentially entirely removed.

[0030] A detecting apparatus 28 detects the level of light emitted bythe reaction of O and NO to form NO₂ and emit light. In a preferredembodiment, the detecting apparatus 28 detects the light through awindow 30. However, there are many ways to arrange the detectingapparatus 28 so that it can detect the emitted light. In a preferredembodiment, the light emissions from the reaction of O and NO to formNO₂ are summed over the wavelength range of 470-770 nm while detectingintensity levels.

[0031]FIG. 5 is a schematic view of a second embodiment of the apparatus10. In this embodiment, a flow of O₂ enters the plasma chamber 12through the plasma chamber inlet 14. Within the plasma chamber 12, theO₂ is dissociated so that O is formed. The flow of O exits the plasmachamber 12 through the plasma chamber outlet 16. The flow of O entersthe stripping chamber 18 as it leaves the plasma chamber 12.Substantially no plasma enters the stripping chamber 18 from the plasmachamber 12, only uncharged gas. A separate input 40 to the strippingchamber 18 is provided to supply a flow of NO to the stripping chamber18.

[0032] In the embodiment shown in FIG. 5, the window 30 and detectingapparatus 28 are placed close to the NO input, to aid detection of thelight emitted in the reaction of O and NO. The detecting apparatus 28detects the intensity of light emitted by the reaction of O and NO toform NO₂, just as in the embodiment shown in FIG. 4.

[0033]FIG. 6 is a schematic view of a third embodiment of the apparatus10. In this embodiment, the reaction of NO and O to produce NO₂ andlight does not occur within the stripping chamber 18. In thisembodiment, a flow of O₂ enters the plasma chamber 12 and is dissociatedso that a flow of O enters the stripping chamber 18, while substantiallyno plasma enters the stripping chamber 18. Substantially no NO exists inthe stripping chamber 18, so the level of light emitted inside thestripping chamber 18 from the reaction of O and NO to form NO₂ isessentially zero. As a flow of O leaves the stripping chamber 18 throughthe stripping chamber outlet 22, it enters a downstream channel 60. Aflow of NO in introduced into the downstream channel 60 through an inlet62 into the downstream channel 60. In the downstream channel 60, theflow of NO and the flow of O react to produce NO₂ and light. This lightin the downstream channel 60 is detected by the detecting apparatus 28.Preferably, the detecting apparatus detects the light through a window30 in the downstream channel 60, although there are many ways to arrangethe detecting apparatus to detect the emitted light. The window 30 anddetecting apparatus 28 are preferably located close to the inlet 60 intothe downstream channel 60 to aid in detecting the light emitted in thereaction of O and NO.

[0034]FIG. 7 is a graph 70 showing the intensity of light detected fromthe reaction of O and NO during a preferred embodiment of thephotoresist stripping process. The intensity levels on the graph 70represent the summation of the intensity of light over the range of470-770 nm. At a first time 72, the photoresist stripping process hasnot begun and the light detected is at a low level. At a time 74, thephotoresist stripping process begins, and a flow of O and NO enters thestripping chamber. Some of the O and NO reacts to form NO₂ and emitlight. As can be seen in the graph 70, the intensity of light detectedincreases after O and NO enter the chamber at a time 74 until theintensity reaches a higher level at a time 76. From a time 76 to a latertime 78, much of the O reacts with the photoresist, and is not availableto react with NO. At a time 78, substantially no photoresist remains onthe wafer, and the O is now free to react with the NO. Thus from a time78 to a later time 80, the intensity of light detected greatlyincreases, more than doubling in intensity. This increase signals theendpoint of the photoresist stripping at a time 80. The photoresiststripping process and the flow of NO and O continues until a time 82 toensure essentially all the photoresist has been stripped from the wafer.At a time 82, the flow of O and NO into the stripping chamber ends, andthe intensity of light detected returns to a low level.

[0035] Although only a few embodiments have been described in detailherein, it should be understood that the described method and apparatusmay be embodied in many other specific forms without departing from thespirit or scope of the invention. Therefore, the present examples andembodiments are to be considered as illustrative and not restrictive,and the method and apparatus are not to be limited to the details givenherein, but may be modified within the scope of the appended claims.

What is claimed is:
 1. A method for detecting an endpoint of aphotoresist stripping process, the method comprising the operations of:stripping photoresist from a wafer disposed inside a chamber; detectingan intensity of light emitted; and using a change in the intensity oflight emitted to detect the endpoint of the photoresist strippingprocess.
 2. A method as recited in claim 1, wherein the chamber containssubstantially no plasma.
 3. A method as recited in claim 1, furthercomprising the operation of: introducing a flow of O and a flow of NOinto the chamber.
 4. A method as recited in claim 3, wherein the emittedlight is emitted in a reaction of NO and O to form NO₂.
 5. A method asrecited in claim 3, wherein the O reacts with the photoresist to stripthe photoresist from the wafer.
 6. A method as recited in claim 3,wherein the operation of detecting a change in the intensity of lightemitted further comprises detecting an increase in the intensity oflight emitted from a reaction of NO and O.
 7. A method as recited inclaim 6, wherein the increase in intensity of the light emitted from thereaction of NO and O signals the endpoint of the photoresist strippingprocess.
 8. A method as recited in claim 6, wherein the emitted lightdetected is in the wavelength range of about 470 nm to about 770 nm. 9.A method as recited in claim 6, wherein a wall of the chamber includes awindow, the intensity of the light emitted being detected through thewindow by a light detecting apparatus.
 10. A method as recited in claim1, further comprising the operations of: introducing a flow of O intothe chamber; providing an outlet from the chamber; providing adownstream channel, the downstream channel being in fluid communicationwith the chamber outlet; introducing a flow of NO into the downstreamchannel; and detecting an increase in intensity of the light emittedfrom a reaction of NO and O, wherein the increase in intensity of thelight emitted from the reaction of NO and O signals the endpoint of thephotoresist stripping process.
 11. A method as recited in claim 10,wherein a wall of the downstream channel includes a window, theintensity of the light emitted being detected through the window by alight detecting apparatus.
 12. A method for monitoring a photoresiststripping process, comprising the operations of: providing a strippingchamber with a first inlet and an outlet; providing a wafer at leastpartially coated with a photoresist layer; providing a detectingapparatus operable to detect an intensity of light emitted from areaction of NO and O to form NO₂; disposing the wafer within thestripping chamber; introducing a flow of O into the stripping chamberthrough the first inlet, wherein the O reacts with the photoresist layerto strip the photoresist layer from the wafer; detecting a change in theintensity of light emitted from the reaction of NO and O to form NO₂with the detecting apparatus; and using the change in intensity of lightemitted from the reaction of NO and O to form NO₂ to monitor thephotoresist stripping process.
 13. A method as recited in claim 12,further comprising the operation of introducing a flow of NO into thechamber through the first inlet, wherein an increase in the intensity oflight emitted from the reaction of NO and O to form NO₂ signals theendpoint of the photoresist stripping process.
 14. A method as recitedin claim 13, wherein a wall of the chamber includes a window, theintensity of the light emitted being detected through the window by alight detecting apparatus.
 15. A method as recited in claim 12, furthercomprising the operation of introducing a flow of NO into the chamberthrough a second inlet, wherein an increase in the intensity of lightemitted from the reaction of NO and O to form NO₂ signals the endpointof the photoresist stripping process.
 16. A method as recited in claim15, wherein a wall of the chamber includes a window near the secondinlet, the intensity of the light emitted being detected through thewindow by a light detecting apparatus.
 17. A method as recited in claim12, further comprising the operation of introducing a flow of NO into adownstream channel in fluid communication with the chamber outlet andwherein an increase in the intensity of light emitted from the reactionof NO and O to form NO₂ signals the endpoint of the photoresiststripping process.
 18. A method as recited in claim 17, wherein a wallof the downstream channel includes a window, the intensity of the lightemitted being detected through the window by a light detectingapparatus.
 19. A method for detecting an endpoint in a process ofstripping photoresist from a wafer, the method comprising the operationsof: performing a first reaction of a first gas and the photoresist tostrip the photoresist from the wafer, the first reaction emitting firstlight having a first intensity; performing a second reaction between asecond gas and the first gas supplied for the first reaction and notreacted with the photoresist, upon completion of the first reaction thesecond reaction emitting second light having a second intensity thatdiffers substantially from the first intensity; and detecting theintensities of the first and second lights to provide an indication asto when to stop the stripping process.
 20. A method as recited in claim19, further comprising the operation of: dissociating O₂ to supply thefirst gas for the first reaction.
 21. A method as recited in claim 19,further comprising the operation of: collecting the first gas that wassupplied to the stripping chamber and that did not react with thephotoresist; and supplying the second gas to the collected first gas tofacilitate performing the second reaction.
 22. A method as recited inclaim 21, wherein: the operation of supplying the second gas tofacilitate performing the second reaction also emits a range ofwavelengths of the second light; and the detecting operation detects theintensities of all of the wavelengths of the range to provide theindication as to when to stop the stripping process.
 23. A method fordetecting an endpoint in a process of stripping photoresist from awafer, the method comprising the operations of: performing the strippingprocess by a first reaction of O and the photoresist to strip thephotoresist from the wafer, first light having a first intensity beingemitted during the first reaction; supplying NO to facilitate an endpoint detection process between the O supplied to the stripping chamberand not reacted with the photoresist and the NO, upon completion of thefirst reaction the end point detection process emitting second lighthaving a second intensity that differs substantially from the firstintensity; and detecting the intensities of the first light and thesecond light to provide an indication of the endpoint.
 24. A method asrecited in claim 23, further comprising the operations of: dissociatingO₂ to form the O for the first reaction.
 25. An apparatus as recited inclaim 23, further comprising the operations of: separately from thefirst reaction, receiving the O supplied for the first reaction and notreacted with the photoresist; and supplying the NO to the received Osupplied for the first reaction and not reacted with the photoresist tofacilitate, separately from the first reaction, the end point detectionprocess between the NO and the O supplied to the stripping chamber andnot reacted with the photoresist.
 26. An apparatus as recited in claim23, wherein: the end point detection process emits the second lighthaving a range of wavelengths; and the detecting operation detects theintensities of all of the wavelengths of the range to provide theindication of the endpoint.